As DC current is passing through a lead acid battery it charges the battery by changing PbSO4 to Pb on the negative electrode and PbSO4 to PbO2 and on the positive electrode. Any excess current electrolyzes water to 2:1 stoichiometric mixture of hydrogen and oxygen
. That mixture is highly explosive by itself, it does not need air. If battery bubbles due to charging it is either boiling hot making water vapor or produces the stoichiometric mixture of hydrogen and oxygen
. I cannot explain “concrete phenomena” but potential of explosion of overcharged battery is definitely probable.